1. Field of the Invention
This invention relates generally to current limiting circuits and, more particularly, to a current limiting circuit which does not employ a series sense resistor for sampling the output current thus achieving a better input-output voltage differential and/or a greater output voltage swing.
2. Description of the Prior Art
Circuits such as voltage regulators require means for limiting current in the event that the circuit's output becomes shorted. A typical voltage regulator circuit includes a series pass transistor which is controlled by the output of an amplifier. The transistor output is then fed back to an input of the amplifier. If the output were to become shorted without any current limiting circuitry, the amplifier could generate an output which would turn the output transistor on to an extent which could be damaging.
In the past, this problem has been avoided by placing a current limit resistor in series with the series pass transistor to sample the current. As an example, the base and emitter terminals of a current limit transistor is connected across a resistor in the emitter of the series pass transistor and the collector of the current limit transistor is coupled to the base of the series pass transistor. In this manner, if the circuit output is shorted, the circuit limit transistor would turn on when the voltage drop across the resistor reached the base-emitter voltage (V.sub.BE) of the current limit transistor and would thereby divert current away from the base of series transistor. Thus, the output current could not exceed V.sub.BE /R where R is the resistance of the resistor.
There are several problems associated with the use of a resistor in the emitter path of the output transistor to sample the current for current limiting purposes. First, there will always be a voltage drop across the resistor even when the circuit is not operating in the current limit mode; i.e., when the voltage drop across the resistor does not exceed the base-emitter voltage of the current limit transistor (typically 0.7 volts). This increases the input-output voltage differential resulting in increased power dissipation. Further, in circuits such as operational amplifiers, it is desirable that the output exhibit as large a voltage swing as possible. The voltage drop across the series resistor limits the output voltage swing.
There has been an attempt to avoid these problems by employing current limiting circuits which are rendered operative in response to a voltage which is a fraction of the base-emitter voltage. Thus, the voltage drop across the resistor is substantially less. However, such attempts have resulted in circuitry which is more complex and costly than conventional approaches.